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[patch] mm, oom: add comment for why oom_adj exists

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  [patch] mm, oom: add comment for why oom_adj exists David Rientjes <rientjes@google.com> - 2015-11-04 21:40 +0100
    Re: [patch] mm, oom: add comment for why oom_adj exists Michal Hocko <mhocko@kernel.org> - 2015-11-05 11:30 +0100
      Re: [patch] mm, oom: add comment for why oom_adj exists David Rientjes <rientjes@google.com> - 2015-11-05 22:30 +0100

#1262576 — [patch] mm, oom: add comment for why oom_adj exists

/proc/pid/oom_adj exists solely to avoid breaking existing userspace 
binaries that write to the tunable.

Add a comment in the only possible location within the kernel tree to 
describe the situation and motivation for keeping it around.

Signed-off-by: David Rientjes <rientjes@google.com>
---
 fs/proc/base.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/fs/proc/base.c b/fs/proc/base.c
--- a/fs/proc/base.c
+++ b/fs/proc/base.c
@@ -1032,6 +1032,16 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
 	return simple_read_from_buffer(buf, count, ppos, buffer, len);
 }
 
+/*
+ * /proc/pid/oom_adj exists solely for backwards compatibility with previous
+ * kernels.  The effective policy is defined by oom_score_adj, which has a
+ * different scale: oom_adj grew exponentially and oom_score_adj grows linearly.
+ * Values written to oom_adj are simply mapped linearly to oom_score_adj.
+ * Processes that become oom disabled via oom_adj will still be oom disabled
+ * with this implementation.
+ *
+ * oom_adj cannot be removed since existing userspace binaries use it.
+ */
 static ssize_t oom_adj_write(struct file *file, const char __user *buf,
 			     size_t count, loff_t *ppos)
 {
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#1263073

On Wed 04-11-15 12:32:14, David Rientjes wrote:
> /proc/pid/oom_adj exists solely to avoid breaking existing userspace 
> binaries that write to the tunable.
> 
> Add a comment in the only possible location within the kernel tree to 
> describe the situation and motivation for keeping it around.

I am not sure this is really needed but it certainly is not harmful.
If this is a way to suppress any attempts for changes like
http://lkml.kernel.org/r/1f80189385e540c2a5b2747a7a265d8c%40SHMBX01.spreadtrum.com
then it does not explain why those are not desirable.

> Signed-off-by: David Rientjes <rientjes@google.com>
> ---
>  fs/proc/base.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -1032,6 +1032,16 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
>  	return simple_read_from_buffer(buf, count, ppos, buffer, len);
>  }
>  
> +/*
> + * /proc/pid/oom_adj exists solely for backwards compatibility with previous
> + * kernels.  The effective policy is defined by oom_score_adj, which has a
> + * different scale: oom_adj grew exponentially and oom_score_adj grows linearly.
> + * Values written to oom_adj are simply mapped linearly to oom_score_adj.
> + * Processes that become oom disabled via oom_adj will still be oom disabled
> + * with this implementation.
> + *
> + * oom_adj cannot be removed since existing userspace binaries use it.

This is a bit strong wording. I think the knob can be removed in the future.

* oom_adj is kept for compatibility reasons. There are still few
* projects which use oom_adj only. We have tried to convert all of them
* which could be found but it will take some time until all those changes
* bubble up to all users. We might try to remove the knob in few years
* if the situtation changes.

> + */
>  static ssize_t oom_adj_write(struct file *file, const char __user *buf,
>  			     size_t count, loff_t *ppos)
>  {

-- 
Michal Hocko
SUSE Labs
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#1263563

On Thu, 5 Nov 2015, Michal Hocko wrote:

> > diff --git a/fs/proc/base.c b/fs/proc/base.c
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1032,6 +1032,16 @@ static ssize_t oom_adj_read(struct file *file, char __user *buf, size_t count,
> >  	return simple_read_from_buffer(buf, count, ppos, buffer, len);
> >  }
> >  
> > +/*
> > + * /proc/pid/oom_adj exists solely for backwards compatibility with previous
> > + * kernels.  The effective policy is defined by oom_score_adj, which has a
> > + * different scale: oom_adj grew exponentially and oom_score_adj grows linearly.
> > + * Values written to oom_adj are simply mapped linearly to oom_score_adj.
> > + * Processes that become oom disabled via oom_adj will still be oom disabled
> > + * with this implementation.
> > + *
> > + * oom_adj cannot be removed since existing userspace binaries use it.
> 
> This is a bit strong wording. I think the knob can be removed in the future.
> 

Perhaps you are my optimistic than I am, but I would think it would be 
difficult to remove a tunable that requires binaries to be re-built to 
avoid.  That was Linus's primary objection, IIRC.  If an application fails 
to oom disable itself because it still writes to oom_adj, the results 
could be a system wide failure.  There are workarounds to that if you have 
root, but I don't think we're in a position to remove it in the near 
future.  I think the comment is clear why it cannot be removed right now 
and its current implementation.

Converting software that writes to oom_adj to use oom_score_adj instead is 
still a worthwhile goal, though, since they'd be using the semantics of 
the effective policy.
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